1. Field of the Invention
This invention relates to asynchronous transfer mode (ATM) communication systems generally, and, more specifically, to multi-casting devices and processes for providing low/medium speed subscriber interfaces.
2. Description of the Related Art
Current implementations in a network using the concept of point to multi-point connection, frequently called multi-casting, or alternatively, broadcasting call from this point forward, as applied to medium speed subscriber interface. Medium speed interface modules are connected to a plurality of links, which are connected to medium speed subscribers using the point to multi-point connection concept. These networks are divided into a lower layer block with a physical layer function and a higher layer block providing an asychronous transfer mode layer function. The medium speed interface modules serve as interfaces between the higher and lower layer blocks. User interface subscriber assemblers that use routing tables containing virtual path identifier and virtual channel identifier conversion values in order to provide subscriber multicasting, are connected to asychronous transfer mode switches. Each port of the lower layer block within the medium speed interface module is connected to a plurality of links, and each of those links has a separate, discrete routing table containing virtual path identifier and virtual channel identifier conversion values that must be used for multicasting each of the ports. The medium speed interface modules bypasses multicasting cells received from the asynchronous transfer mode switch to the lower layer block and, in turn, the lower layer block performs virtual path/virtual channel conversion by using the port routing tables.
Recent efforts such as the ATM Switching Element And Method Having Independently Accessible Cell Memories of B. D. Holden, U.S. Pat. No. 5,583,861 (and repeated by B. D. Holden in his divisional U.S. Pat. No. 5,570,348), suggest construction of a switch fabric with a plurality of asynchronous transfer mode switches, and one routing table circuit for each physical connection from the switch fabric to a work station, maintaining a marked interrupt linked list for storing information about which of its virtual channels are experiencing congestion, while the Shared Buffer Memory Switch For An ATM Switching System And Its Broadcasting Control Method by K. Yamada, U.S. Pat. No. 5,610,914, suggests a scheme in which the routing information may also be extracted by a shared buffer memory controller instead of being received from a cell multiplexer, in order to simplify the configuration of the cell multiplexer and thereby improve the processing speed.
Traditionally, as noted by J. S. Turner in Packet Switch With Broadcasting Capability For ATM Networks, U.S. Pat. No. 5,229,991, one of the problems encountered in implementing the addition of broadcasting to a point-to-point switching fabric is the inordinate size of the memory required for each broadcast translator circuit and the head end as a whole. R. Barnett, in ATM Cell Switch Suitable For Multicast Switching, U.S. Pat. No. 5,436,893, suggests an ATM cell switch for multicast switching with a plurality of link controllers each connected via an input port and an output port to a dynamic crosspoint switch. Each link controller has a plurality of external data links, and is arranged to read VPI/VCI entries for each data cell received from an external link. Ostensibly, this simplifies the switch fabric by avoid the need to maintain VPI/VCI mapping tables in the switch fabric.
We have noticed that contemporary implementation of point to multipoint connection necessitates the use of multiple small memories, at a concomitant requirement of substantial board space, with the accompanying cost. Moreover, maintenance of this type of implementation incurs part supply problems. Furthermore, we have found that this type of implementation is not efficient because the number of subscriber links can not be flexibly handled.